Coloring stainless steel



nited States PatetitfOice COLORING STAINLESS STEEL Roy C. Bongartz, Pittsburgh, and Jack M. Beigay, New Kensington, Pa., assignors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Filed Dec. 16, 1957, Ser. No. 702,828

1 Claim. (Cl. 204--56) i This invention relates to the coloring of stainless steel and in particular to a method of providing a variety of permanent colo'rs on bright and lustrous stainless steel surfaces.

Stainless teels are conventionally provided with highly attractive, bright and lustrous metallic finishes that are 2,957,812 Patented Oct. 25, 19.60

vide a method of coloring stainless steel whereby the colored surface is relatively permanent and may not be easily eroded from the surface.

Other objects and advantageous features of the present invention will be obvious from the following description.

, steels, the chromium-manganese stainless steels and the ideal for such applications as automobile trim, building trim and kitchenware. However, there has long been a desire among the fabricators and users of stainless steel to provide attractively colored stainless steel for many applications. A metho'd of coloring stainless steel, particularly one that does not destroy thelbright and lustrous finish of the stainless steel, would greatly enhance the beauty and usefulness of many stainless steel articles. There exist many known methods for providing-a colored surface to stainless steel. Some of these methods in volve the immersion of stainless steel in molten salts, such as molten dichromates and nitrates, which has the distinct disadvantage of requiring exceptionally high temperatures. Other methods are known such as treatment in sulfuric acid-dichromate solutions and phosphoric acid solutions, and electrolytic treatments in acid solutions. These methods, however, are capable only of producing a single color on the treated metals, they are dif-. ficult to control for reproducing exact shades, the'colo'red surface is generally detrimental to the luster of the metal chromium-nickel-manganese stainless steels. Steels that contain generally from about 10% to chromium and less than about .25% carbon may be regarded as stainless steels insofar as the present application is concerned. The method of the present invention is not applicable to low alloy or non-stainless grades of steel in that the reaction of these metals under the conditions of the present method provides a steely gray, porous, unattractive, loosely adhering oxide film. The application of the present method to stainless steel under the conditions set forth is believed to produce an extremely thin, even, transparent oxide fihn on the surface thereof which will provide interference patterns that impart colors to the. stainless steel surface, but which do not destroy I the naturalv luster of the metal.

metal hydroxides may be employed to either partly or and the coating which provides the color is not tenacious and can be easily removed by a mild abrasive action. Another method sometimes employed to color stainless steels is to heat tint stainless article in an oven. Colors provided by this process are not easily reproduced, the method does not provide much of a variety of colors and the luster of the stainless steel surface is adversely affected.

It has now been discovered that by the method of the present invention stainless steels may be given a variety of colors that do not adversely affect the lustre of the metal, are substantially reproducible and adhere satisfactorily to the metal surface.

In general, the present invention relates to a method of providing highly desirable colors on the surface of stainless steels. Stainless steel is immersed in an aqueous solution that contains an alkali metal hydroxide, preferably potassium hydroxide, and an electric current is impressed through the solution with the stainless steel as the anode. The current density must be maintained witha range of from about A: ampere per square foot to not more than 20 amperes per square foot and preferably within the range of from A ampere per square foot to 10 amperes per square foot. The temperature is preferably within the range of room temperature to the boiling point of the solution, and the preferred concentrawholly replace the potassium hydroxide. For example, sodium and lithium hydroxide may be employed in substantially the same concentrations as potassium hydroxide.

Differences in the coloring efiects of hydroxides other than potassium hydroxide may be compensated for by adjusting the current density, strength of solution and temperature during treatment. The concentration of the alkali metal hydroxide is not extremely critical in that temperature, time and current density may be adjusted to compensate for differences in concentration levels. However, practical considerations in obtaining satisfactory results within a reasonable time require at least 10% by weight of the hydroxide to be present. Also, concentrations in excess of 70% by weight are impractical due to the lower chemical activity of such a bath. Type 430 stainless steels that exhibit a relatively bright and lustrous finish (automotive finish) have been given a variety of satisfactory permanent colors by the method of this invention while empolying a concentration of from about 10% to 50% KOH with the balance water. Up to 70% KOH in water has proved to be highly satisfactory in coloring Types 301 and 302 stainless steel. In general, it appears that use of the higher concentrations (40% to 60%,) will result in a wider range of colors than is obtainable from the lower concentrations (10% to less than 40%).

After coloring is completed the stainless steel is Withdrawn from thehydroxide solution, rinsed free of the hydroxide salts, preferably in Water, and dried.

The solution temperature is not critical in that reproducibility may be obtained by adjustment of current density, concentration and time. However, once again the temperature of the solution must be within a practical range of from about room temperature (60 F.) to the boiling temperature of the solution being employed. Alkali metal hydroxide solutions containing about 70% of the hydroxide salt are generally solid at rodm temperature and must be heated from 350 F. to 400 F. to provide a liquid that is necessary to carry out the method of the present invention. It is preferred to maintain the solution temperature within a. range of from about 60 F. to 400 F. depending on the concentration of the hydroxide in the solution.

The current density in the process of the present invention is a critical factor in that when over about 20 amperes per square foot are employed the action of the treatment tends to etch the surface of the stainless steel and destroy the bright and lustrous finish. Also, any coloring attained at such high current densities is susceptible to being easily removed by mild abrasions such as wiping, etc. Current densities under about /a ampere per square foot require excessive times and temperatures to provide any coloring at all and will not provide a variety of the desired permanent shades of color. Best results have been obtained by employing about A: to amperes per square foot. Colors nearly identical to others may be obtained by changing the concentrations of the electrolyte used or current densities used, although a change in these variables will usually cause a change in the intensity or shade of the color obtained in one case as opposed to another.

In general, current densities above amperes per square foot will cause the colors formed on Type 430 stainless steel in fifteen minutes, or longer, to be rather loosely held as compared to the fairly tight adherence of colors formed at 10 amperes per square foot or below. Such loose colors are all brown-red of slightly varying shades.

The time factor involved in coloring by employing the method of the present invention is not extremely critical; however, practical considerations require a time of less than an hour and at least one minute. By employing a given concentration of solution, temperature, time and current density, shades of yellow, gold, brown, red, green and blue may be obtained on bright and lustrous finishes. Such colors and shades of colors may be reproduced by proper adjustment of the variables involved.

The method of the present invention may be applied to color the surface of any stainless steel regardless of the finish so long as a clean surface substantially free of oxide scale is provided. The method may be successfully applied to a cold rolled, annealed and pickled finish such as is commonly referred to in the steel indust y as a No. 2 finish. However, a particularly attractive and desirable colored finish is obtained when the method is employed to color bright and lustrous finishes such as a No. 4 sheet finish or brighter finishes. A No. 4 finish is well known in the steel industry as one that has been obtained by polishing metal with 120 to 150 grit abrasive material. Finer finishes such as a No. 7 or No. 8 finish are obtained by further polishing with finer abrasive ma- :terials and by buffing with cloth wheels. Stainless steel :strip that has been cold rolled, annealed and pickled free of scale may be given a final rolling through extra smooth rolls to present a finish commonly known as automotive finish due to its frequent application as automobile trim. Such a finish presents a particularly desirable and at- :tractive surface when colored by the method of the present invention. An automobile finish is brighter and :smoother than a No. 4 finish. The method of the present invention is equally applicable to finishes obtained by means other than mechanical such as electropolishing and bright pickling.

The following specific examples are given to illustrate the method of the present invention and do not limit the invention to the exact method and conditions set forth.

Two inch by four inch samples of Type 430 stainless steel strip material that exhibit a bright and lustrous finish (automotive finish) were thoroughly cleaned and immersed in aqueous potassium hydroxide solutions and connected as itheanode .to asuitable power source, any suit:

able cathode being also connected to the power source and immersed in the solution. The concentration of solut-ion, temperature, current density, times and colors obtained are given below:

KOH Solution Current Color Density, Time Concen- Tempera- Amp./ (Mm) tration ture Sq. Ft (percent) F.)

Pale yellow gray 20 2% 5 Brownish gold. 20 2% 5 Gold 20 175 2 15 20 200 2% 10 Reddish gold- 20 150 4% 15 Reddlsh gold- 20 175 4% l0 r0W11. 20 150 9 10 Brown 20 175 9 5 Green-go 20 200 9 10 Red green- 30 175 2% 10 Green gold-.. 30 175 2% 10 Reddish gold- 30 200 2% 15 Light brown.-. 30 125 2% 10 e 30 175 2%; 15

Commercial 3 inch house numbers fabricated from Type 430 automotive finish strip were given a variety of pastel shades in a 30% KOH solution under the following conditions:

House Current Time Temperature Number Density, (Min.) F.) Color Amp/Sq. Ft.

7 4 Green 2 10 Red. 9 2 169 Gold.

17 1 5 158 Blue.

3 5 156 Blue green.

Type 301 automotive finish wheel covers were colored in 40% KOH solution. A few examples of the colors obtained are given with the conditions used.

The cathode employed in each of the examples given hereinbefore was constructed of Type 316 stainless steel.

As is illustrated by the above examples, the method of the present invention provides a wide variety of permanent reproducible, attractive colors.

We claim:

The method of coloring stainless steel containing from 10% to 35% chromium comprising, removing all oxide scale from the surface of said steel and imparting thereto at least a No. 2 surface finish, immersing said steel in an aqueous solution that consists essentially of at least one of the alkali metal hydroxides within the range of from about 10% to about 70% at a temperature of from about 60 F. to the boiling temperature of said solution, passing an electric current of a predetermined current density within the range of from about Vs ampere per square foot to about 20 amperes per square foot for a predetermined time within the range of from 1 hour to 1 minute while maintaining said steel as the anode therein so as to effect a predetermined color on the surface of said steel without destroying the surface finish, withdrawing said steel from said solution, rinsing said steel with water and drying.

References Cited in the file of this patent UNITED STATES PATENTS 621,084 Hollis Mar. 14, 1899 1,954,473 Dunn Apr. 10, 1934 1,978,151 Van Mater Oct. 23, 1934 2,827,425 McGlasson et al Mar. 18, 1958 

